A volcano erupts when magma from the underground lave chamber of a volcano gets so much pressure that it has to let up and out, and that's what happens: The magma, now called lava, leaves its chamber and BOOM! the volcano erupts with an explosion! It's a fascinating thing when a volcano erupts. On the other hand, haven't most of us heard of volcanoes like Mt. St. Helens, Mt. Vesuvius, Mt. Pinatubo, etc.? The reason they're natural disasters is because they erupted with enough force to kill many.